Fixing cellotex (or alternative) behind stud walls

You just cut it very slightly too large, and it will wedge happily between the studs.

David

And fill any gaps with expanding foam - which can also be used to glue boards in place.

Where there's a likelihood of wind howling around (like a dormer :-), I've put a thick bead of silicone round the inside face of the celotex before fitting it so there's less risk of draughts next to the warm side negating the insulation.

and cover the studs with the special foil tape, to vapour seal everything - and draught proof too.

I don't like windows in bathrooms, they are a pain and unnecessary this day and age, my advice ie to remove it and replace it with a high insulation panel of some kind not forgetting to build in a fan, then insulate whole wall as others have suggested. Don

SORRY, THIS SHOULD HAVE BEEN IN THE THREAD " Insulating a wall" Don

Not in this case, as the plasterboard is already in place and the OP's accessing the studs from behind.

David

Chicken wire and 100mm rockwool is just as effective. Doesn't look pretty but you aren't going to look at it.

Possibly, if you include a vapour barrier too.

OTOH, it's also twice the thickness. In many cases this is a sloping ceiling, which could be the equivalent of making the room 3" smaller.

Ah - you (the OP?) said stud walls - the rockwool goes between the studs which are there anyway, so no encroachment on the room. For a roof, there has to be airflow between the rafters so Celotex. Personally I believe the whoile vapour barrier thing to be a bit of an urban myth, YMMV